For smooth change-ratio shifts in automatic transmissions, it is necessary that the off-coming or disengaging clutch precisely releases when the on-coming or engaging clutch has reached sufficient capacity to carry the required torque. In the sequence of events during a typical shift, this transition happens at the end of the torque phase of a shift. At this point the torque in the off-coming clutch is reversed and this clutch must be released to prevent a tie-up or excessive overlap in the transmission gearing and a resulting sagging shift. Conversely, an early release of the off-coming clutch before the on-coming clutch engages will result in an engine flare and a harsh and jerky shift.
To provide for shifts in which sagging and harshness are minimized, various shift transition mechanisms, conditioned by inputs from a wide range of controls, have been utilized to selectively frictionally hold or drivingly connect different components of planetary gear sets found in most transmission designs. Among these mechanisms are one-way roller or free-wheeler type clutches which automatically hold in one direction to provide reaction for transmission gearing and which release automatically on torque reversal.
Such one-way clutches are generally considered to be the most highly efficient and effective control mechanisms since they are direction sensitive and are inherently operationally efficient and accurate. However, since one-way clutches have torque capacity in only one direction, an additional selectively engageable friction clutch must be provided to inhibit overrun for purposes such as engine braking. Furthermore, if used as an intermediate clutch such one-way clutches must be free to turn in an opposite direction in low gear. This necessarily requires another clutch in series with the intermediate one-way clutch drive for intermediate to low range shifts. Such additional clutch construction adds to the overall length of the transmission so that it cannot be readily used in many instances where a short length transmission is required. For example, transverse engine-transmission power packages in crowded forward compartments of front wheel drive vehicles require foreshortened transmissions.
In addition to one-way clutches, single or dual wrapped friction bands are often used to selectively lock rotation of components of planetary gearing of the transmission for establishing a ratio. While such bands are effective holding devices and characteristically have a low torque capacity in a de-energized direction so that tie ups with an on-coming clutch are minimized, they are difficult to control and cannot be relied on to produce consistent, high quality ratio shifts.
A third type of friction drive device known as multiple plate clutches have been utilized for controlling planetary transmission gearing. However, smooth shifting off of such multiple plate clutches necessitates intricate and costly controls so that use is restricted to a limited number of applications. Also, Since the shift quality from such multi-plate clutches is often inconsistent, excessive clutch plate wear and poorly timed shifts and engine flair are often experienced.
To improve shifts in a multi-ratio change speed transmission, the present invention incorporates the best operational characteristics of the prior shift mechanisms and particularly those of a one-way roller clutch into a multi-mode friction clutch. This clutch is a multi-mode unit that can effectively function in a one way drive mode of operation to hold a reaction member of planetary gearing in one direction to smoothly establish a ratio and release automatically at torque reversal in the manner of a one-way roller or free-wheeler clutch for smooth shifts. The multi-mode clutch of this invention can also function in a two way locking mode to hold a component of the planetary gearing in both drive and coast direction. With coast locking, the multi-mode clutch provides engine braking. Additionally, the multi-mode clutch of this invention can be fully released for rotation in either direction allowing other friction devices in the transmission to establish the drive ratios. This clutch can be employed to selectively connect and release two rotatable members as needed in some transmission designs.
In a preferred embodiment of the present invention, the multi-mode clutch incorporates an annular piston-like, torque sensitive reaction plate that is associated with a hydraulically actuated piston movable by apply pressure fed to an apply chamber, and that carries friction plates which are interleaved with friction plates operatively connected to a planetary gear set member for establishing a drive through the transmission. The torque sensitive reaction plate is moved by a torque responsive wedging actuator and by the opposing force of pressure in the apply chamber to control the area of an exhaust opening leading from the chamber thereby controlling the apply pressure within the chamber for the piston so that the capacity of the clutch is controlled to improve transition in ratio shifts.
With the multi-mode clutch of this invention, compactness is provided so that friction bands and free wheeler and other overrun clutch devices such as sprag clutches can be eliminated or reduced in number. This allows substantial foreshortening of overall transmission length such as by 50-60 mm in some designs, for example, so that it can be readily used in a transversely mounted power package for vehicles. With the multi-mode clutch of this invention, costs are lowered while design flexibility is enhanced since the multi-mode clutch is particularly effective in utilizing available space within a transmission case.
The clutch of this invention is applicable to both stationary and rotating type clutches, accordingly, the term clutch as used in this specification defines a friction device that selectively brakes and holds a member to ground or selectively drivingly connects two members for rotation together. The multi-mode clutch of this invention can be readily employed to convert existing transmissions from a band to a free wheeler shift without major alterations.
These and other features, objects and advantages of this invention will become more apparent from the following detailed description and drawings in which: